Combined cycle with steam cooled gas turbine

ABSTRACT

In a method of operating a combined cycle system including a gas turbine, a steam turbine and a multi-pressure heat recovery steam generator, an improvement includes supplying gas turbine cooling duty steam from a high pressure section of the steam turbine and from an intermediate pressure evaporator in the multi-pressure heat recovery steam generator, conducting the gas turbine cooling duty steam to the gas turbine for cooling hot gas turbine parts, and then returning the gas turbine cooling duty steam to an intermediate pressure section of the steam turbine. In a start-up procedure, steam is extracted from a first pass of a high pressure superheater in the multi-pressure heat recovery steam generator, mixed with steam discharged from the high pressure superheater and then supplied to the gas turbine cooling duty system. In this same start-up procedure, the gas turbine cooling duty steam is returned to the system condenser, bypassing the intermediate pressure section of the steam turbine. Related apparatus is also disclosed.

RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.08/417,426 filed Apr. 5, 1995 which is a continuation-in-part ofapplication Ser. No. 08/161,070 filed Dec. 3, 1993 (U.S. Pat. No.5,428,950) which in turn, is a continuation-in-part of application Ser.No. 08/145,633 filed Nov. 4, 1993 (U.S. Pat. No. 5,412,937).

TECHNICAL FIELD

This invention relates to a combined cycle power generation system inwhich exhaust gases from a gas turbine are recovered in an unfired,multi-pressure, heat recovery steam generator, and in which steam fromthe steam turbine exhaust and from the heat recovery steam generatorintermediate pressure evaporator is utilized to cool the gas turbinestage 1 and 2 nozzles and buckets.

BACKGROUND PRIOR ART

In typical combined cycle power generation systems, cooling of gasturbine high temperature components and the accompanying steam cycle areusually of the following types:

(1) Air Cooled Gas Turbine--The gas turbine high temperature componentsare cooled by air extracted or conducted from other components in thecycle. The steam cycle and the gas turbine coolant streams are notintegrated.

(2) Water Cooled Gas Turbine--The gas turbine high temperaturecomponents are cooled with water in the liquid phase. The heat extractedfrom the high temperature gas turbine components is integrated with thecombined cycle steam bottoming cycle. The energy extracted from the hightemperature section of the gas turbine is transported to the lowtemperature portion of the steam cycle to maintain the water in theliquid phase, thus compromising thermal efficiency of the cycle.

(3) Steam Cooled Gas Turbine Integrated Into a Combined Cycle withMultiple Pressure--This cycle uses steam from the low pressure sectionof a multiple pressure combined cycle to cool the high temperaturecomponents of the gas turbine with energy extracted from the gas turbinereturned to the low pressure section of the steam cycle. This system isdescribed in U.S. Pat. No. 4,424,668. The thermal efficiency that can beachieved by this system is inferior to that achievable with thisinvention, however, because, in the '668 system, energy is transportedfrom the high temperature part of the cycle to a low temperature sectionof the cycle for conversion of the heat energy to power.

DISCLOSURE OF THE INVENTION

The present invention integrates advanced technology relating to closedcircuit steam cooled 60 Hz and 50 Hz gas turbines as well as advancedtechnology steam turbines, along with reliable steam cycles usingunfired, multi-pressure, reheat, heat recovery steam generators (HRSGs).In a preferred arrangement, the gas turbine section of the combinedcycle system includes generally a compressor, a combustion system and agas turbine. The steam turbine system includes generally a steam turbineand a condenser, and the steam turbine drives a generator which in turnproduces electrical power. In the preferred arrangement, the gas turbineand steam turbine are coupled to the generator in tandem on a singleshaft.

The gas turbine cooling system in accordance with thiscontinuation-in-part application is integrated into the steam turbinesystem in that steam is supplied from the high pressure (HP) steamturbine exhaust and the HRSG intermediate pressure (IP) evaporator tothe closed circuit system that cools the gas turbine stage 1 and 2nozzles and buckets. The cooling steam is returned to the steam cycle inthe hot reheat line. Thus, the cooling system operates in parallel withthe HRSG reheater.

More specifically, the supply of cooling steam from the exhaust of theHP steam turbine and the HRSG intermediate pressure evaporator drum isdelivered to the gas turbine stationary parts through casing connectionsand to the rotor through a conventional gland with labyrinth seals andappropriate leak-offs. The cooling steam is returned to the steam cycleat the hot reheat line, from which it is admitted to the IP steamturbine.

A start-up steam supply system is also included which extracts steamfrom the HP superheater after the first pass and mixes it with steamfrom the superheater discharge to supply steam to the cooling system atthe required temperature. The steam from the IP drum is also used. Whenthe gas turbine is operating with a start-up steam supply, the coolingsteam is not admitted to the IP turbine, but instead is bypassed to thecondenser through the IP bypass valve and attemperator. The IP bypassvalve is modulated to maintain the pressure of the cooling steam abovethe gas turbine compressor discharge pressure to prevent gas leakageinto the cooling steam, and hence the steam cycle.

During gas turbine start-up, acceleration to rated speed and operationat low load, the gas turbine is cooled by air extracted from thecompressor discharge. The air is filtered prior to supply to the gasturbine cooling system. Cooling air from the gas turbine cooling systemis discharged to the gas turbine exhaust. Appropriate shut-off valvesisolate the cooling system from the steam cycle while it is operatingwith air cooling.

A bypass is provided in the cooling steam circuit immediately upstreamof the steam shut-off valves. This bypass allows steam from the startingsteam supply to pass through the steam lines and to be discharged to thecondenser through the IP bypass valve which warms the steam lines andstabilizes the steam system during starting prior to admitting steam tothe cooling system. The cooling steam shut-off valves are hydraulicallyactuated and included in the trip circuit such that the system istransferred to air cooling immediately upon a normal or emergencyshut-down to purge steam from the cooling system.

While the system can be configured with either forced or naturalcirculation evaporators, natural circulation evaporators are preferred.The HRSG in the embodiment described herein is a typical three pressure,reheat HRSG that is commonly applied in combined cycles. It includes thefollowing features to accommodate the steam cooling system as describedherein.

1. The reheater size is reduced since pan of the reheating is performedby the gas turbine cooling system.

2. The reheater is located in the gas path downstream of the hightemperature section of the HP superheater. Sufficient heat istransferred to the high temperature superheater to reduce the gastemperature entering the reheater such that the reheater can operatewithout steam flow if all of the reheat steam is diverted to the gasturbine cooling system during a transient or other unusual operatingcondition.

3. Control of the HP steam temperature is accomplished by the steamattemperation system which bypasses a section of the HP superheater.This system eliminates the potential for contaminants to enter the steamas can occur with attemperation with feed water. Attemperation steam isextracted after it passes through one pass in the superheater to assurethat it will be dry after the small pressure drop across the steamcontrol valve. Once the gas turbine cooling system is operating in itsnormal mode, with cooling steam supply from the IP evaporator and HPsteam turbine exhaust, all of the HP steam passes through the hightemperature section of the superheater to limit the temperature of thegas entering the reheater.

4. Provisions for extracting steam for the start-up cooling systemsupply from the HP superheater downstream of the first pass.

Supply of high purity steam to the gas turbine cooling system is a keyfeature of the invention. Features included in the system to accomplishthis objective are:

1. All cooling steam is purified by evaporation in a steam drum.

2. HP steam temperature is controlled by steam attemperation.

3. Full flow filtration and demineralization of feed water.

4. Inserting all piping in the HP and cooling steam system duringstandby periods to prevent corrosion. This system is an extension of thenitrogen blanketing system normally included for an HRSG.

5. Application of non-corrosive materials in piping, filters andequipment downstream of the cooling steam shut-off valves.

6. Full flow steam filtration.

In its broader aspects, the present invention thus relates to a methodof operating a combined cycle system including a gas turbine, a steamturbine and a multi-pressure heat recovery steam generator, theimprovement comprising a) supplying gas turbine cooling duty steam froma high pressure section of the steam turbine and from an intermediatepressure evaporator in the multi-pressure heat recovery steam generator;b) conducting the gas turbine cooling duty steam to the gas turbine forcooling hot gas turbine pans, and c) returning the gas turbine coolingduty steam to an intermediate pressure section of the steam turbine.

In another aspect, the invention relates to a combined cycle systemincluding a gas turbine, a steam turbine and a multi-pressure heatrecovery steam generator, the improvement comprising means for supplyinggas turbine cooling duty steam from a high pressure section of the steamturbine and from an intermediate pressure evaporator in themulti-pressure heat recovery steam generator and means for conductingthe gas turbine cooling duty steam to the gas turbine for cooling hotgas turbine pans, and means for returning the gas turbine cooling dutysteam to an intermediate pressure section of the steam turbine.

In still another aspect, the invention relates to a combined cyclesystem including a gas turbine, a steam turbine and a multi-pressureheat recovery steam generator, wherein the improvement comprises a) ameans of cooling the gas turbine hot gas path pans (stationary androtating) during start-up, low loads, and after a unit trip, usingcompressor discharge air; b) a means of transferring to and from aircooling to start-up steam cooling which employs a mixture of HPsuperheater discharge steam, HP superheater bleed steam, and IPevaporator steam to prewarm the steam supply lines and cool the gasturbine hot gas path pans prior to the eventual availability of: c)normal mode cooling steam supply from the IP evaporator and HP turbineexhaust as well as; d) a means of transferring from start-up coolingsteam supply to normal mode cooling steam supply and; e) a means ofreturning the start-up cooling steam to the condenser and the normalmode cooling steam to an intermediate pressure section of the steamturbine.

Additional objects and advantages of the present invention will becomeapparent from the detailed description as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing the manner of separation of FIGS. 1A and1B;

FIGS. 1A and 1B together comprise a schematic flow diagram for amulti-pressure reheat combined cycle system with a steam cooled gasturbine in accordance with this continuation-in-part application; and

FIG. 2 is a schematic flow diagram similar to that illustrated in FIGS.1A and 1B, but modified to show a start-up circuit.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIGS. 1A and 1B, the invention is incorporated in amulti-pressure reheat combined cycle power generation system 10. In thepreferred embodiment, there is included a gas turbine system 12comprising a compressor 18, a combustion system 16 and a gas turbine 14.A steam turbine system 20 includes a high pressure section 22, anintermediate pressure section 24 and one or more low pressure sections26 with multiple steam admission points at different pressures. The lowpressure section 26 exhausts to a condenser 28. The steam turbine drivesthe generator 30 which produces electrical power. The gas turbine 12,steam turbine 20 and generator 30 are arranged in tandem on a singleshaft 32.

The combined cycle system as described herein includes a multi-pressureHRSG 36 which includes a low pressure (LP) economizer 38, an LPevaporator 40, an HP and IP economizer 42, a low pressure superheater44, an IP evaporator 46, an HP economizer 48, an HP evaporator 50, afirst HP superheater section 52, at least one intermediate HPsuperheater section 54, an IP reheater 56 and a final HP superheatersection 58 all arranged substantially as disclosed in parent applicationSer. No. 08/161,070.

Condensate is fed from condenser 28 to the HRSG 36 via conduit 60 withthe aid of pump 62. The condensate subsequently passes through the LPeconomizer 38 and into the LP evaporator 40. Steam from the low pressureevaporator 40 is fed to the LP superheater 44 via conduit 64 and is thenreturned to the low pressure section 26 of the steam turbine 20 viaconduit 66 and appropriate LP admission stop/control valves (not shown).

Feed water with the aid of pump 68 and 69 passes through the HP and IPeconomizers 42 via conduit 70 and 71 and then to the final HP economizer48 via conduit 72. At the same time, steam from the IP evaporator 46leaves the HRSG 36 via conduit 74 to the closed circuit gas turbinecooling system as described further hereinbelow.

Meanwhile, condensate in the final HP economizer 48 is passed to the HPevaporator 50 via conduit 76. Steam exiting the HP evaporator 50 viaconduit 78 through the superheater sections 52, 54 and 58, and is thenreturned to the HP section 22 of the steam turbine 20 by way of conduits80 and 82.

Heat is provided to the HRSG 36 by the exhaust gases from the gasturbine 22, introduced into the RSG 36 via conduit 84, and which exitthe HRSG 36 via stack (not shown) via conduit 86. Optional fuel may beadded to the HRSG 36 via conduit 88.

The normal supply of cooling steam is from the exhaust of the HP steamturbine via conduit 90 and the HRSG IP drum via conduit 74. The coolingsteam from the HRSG IP drum 46 and the exhaust of the HP steam turbine22 join in conduit 92 and is delivered to the gas turbine 14 throughcasing connections and to the gas turbine rotor through a conventionalgland with labyrinth seals and appropriate leak-offs (not shown).Cooling steam heated by the gas turbine cooling duty is then conducteddirectly to the inlet of the IP steam turbine via conduits 94 and 96. Inthe preferred embodiment, this (now heated) cooling steam is mixed withsteam from the reheater 56 via conduit 98 prior to admission to the IPsection 24. A gland leak-off conduit 100 permits a very small amount ofsteam from the gas turbine rotor gland to be returned to the exhaust ofthe IP section 24 of the steam turbine 20.

It has been the practice to control steam temperature in the forwardsection 58 of the superheater 56 by a water spray introduced into anattemperator located downstream of the first pass 52 of the superheaterin order to control the temperature of the steam.

It has been found, however, that the water spray input to theattemperator from, for example, the feedwater pump 68 (similar to thearrangement in FIG. 4 of parent application Ser. No. 08/161,070) can beproblematical if the water contains impurities. As a result, it has beendetermined that steam purified in a steam drum would provide anattractive alternative to the conventional water spray.

Accordingly, the invention here utilizes steam from one end of thesuperheater, i.e., from a pass in the superheater section 52, to control(cool) the temperature of the superheated steam in section 58 at theopposite end. Thus, as shown, conduit 102 carries superheated steam fromthe first pass of superheater section 52 to the steam inlet ofsuperheater 58, at or near the final pass, and ahead of the reheater 56.This attemperating steam is taken outside the HRSG so as not to beexposed to the gas turbine exhaust gas, under the control of valves 104and 110. In other words, the superheated steam extracted via conduit 102is cooler than the superheated steam in the forward section 58. As aresult, the superheated steam entering section 58 is cooler, allowing agreater reduction in the gas turbine exhaust gas temperature by reasonof enhanced absorption of heat into the superheated steam in section 58.This, in turn, provides temperature control of both the superheater andthe reheater 56.

In steam systems operating at low enough pressure that drum steam can bethrottled without entering the moisture (two phase) region, theattemperation steam may optionally be taken directly from the steam drumrather than the first superheater pass. This is shown as conduit 106 andvalve 108, which in this instance would replace conduit 126 and valve104.

It will be appreciated that steam taken from the superheater 52 viaconduit 126 has been substantially purified in the drum of the highpressure evaporator 50, so that the contamination problem attendantconventional attemperators has been substantially eliminated.

Turning now to FIG. 2, a start-up steam supply circuit is illustratedwherein steam is extracted from the HP superheater 52 after the firstpass via conduits 102 and 116 and mixes with steam from the superheaterdischarge conduit 80, temperature control valve 118 and flow controlvalve 120, to supply steam to the gas turbine cooling system via conduit90 at the required temperature. Steam from the IP drum 46 (via conduit74) is also utilized in the gas turbine cooling system as describedhereinabove. A remaining portion of the HP steam from superheatersection 58 may, on start-up, be directed via conduit 128 and bypassvalve 130 to the attemperator 132 and then back to the condenser 28 viaconduit 134.

When operating with the start-up steam supply, the return cooling steamis not admitted to the IP turbine 24. Rather, the return cooling steamin conduit 94 is bypassed to the condenser 28 through the IP bypassvalve 122 and attemperator 124 and conduit 126. The IP bypass valve 122is modulated to maintain the pressure of the cooling steam above the gasturbine compressor discharge pressure to prevent gas leakage into thesteam cycle. A bypass 141 is also provided in the cooling steam circuitimmediately upstream of the steam shut off valve 140. This bypass warmsthe steam lines and stabilizes the steam system during starting, priorto the admission of steam to the cooling system. In this regard, thecooling steam shut-off valves are hydraulically actuated and areincluded in the trip circuit such that the system is transferred to aircooling immediately upon a normal or emergency shut-down to purge steamfrom the cooling system.

During gas turbine start-up, and acceleration to rated speed andoperation at low load, the gas turbine 12 is cooled by air extractedfrom the compressor discharge via conduit 136. The air is filtered at138 prior to supply to the cooling system. This cooling air from the gasturbine is discharged to the gas turbine exhaust via conduit 137.Appropriate shut-off valves isolate the cooling system from the steamcycle while it is operating with air cooling.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. In a method of operating a combined cycle systemincluding a gas turbine, a steam turbine and a multi-pressure heatrecovery steam generator, wherein gas turbine exhaust is used in theheat recovery steam generator for reheating steam from the steamturbine, the improvement comprising supplying gas turbine cooling dutysteam from a high pressure section of the steam turbine and from anintermediate pressure evaporator in the multi-pressure heat recoverysteam generator, conducting the gas turbine cooling duty steam to thegas turbine for cooling hot gas turbine parts, and then returning thegas turbine cooling duty steam to an intermediate pressure section ofthe steam turbine.
 2. The method of claim 1 wherein, early in start-up,prior to the availability of any cooling steam, the gas turbine hotstationary and rotating parts are cooled with filtered compressordischarge air.
 3. The method of claim 1 wherein, during start-up, asstart-up cooling steam is being established, the start-up cooling steamis bypassed around the intermediate pressure section of the steamturbine to warm the piping and valves.
 4. The method of claim 1 wherein,during start-up, steam is extracted from a first pass of a high pressuresuperheater in the multi-pressure heat recovery steam generator, mixedwith steam discharged from the high pressure superheater and thensupplied to the gas turbine cooling duty system.
 5. The method of claim1 wherein, during staff-up, the gas turbine cooling duty steam isreturned to a condenser, bypassing the intermediate pressure section ofthe steam turbine.
 6. The method of claim 1 wherein the heat recoverysteam generator includes a high pressure superheater and wherein themethod includes controlling temperature in the high pressure superheaterby:a) extracting superheater steam from a first pass of the superheater;b) conducting the extracted steam outside the heat recovery steamgenerator; and c) reintroducing the extracted steam at the inlet of thefinal superheater, which occurs first in the gas path, ahead of allother HRSG heat transfer surfaces.
 7. The method of claim 1 wherein thegas turbine cooling duty steam is mixed with steam from a reheater inthe heat recovery steam generator upstream of the intermediate pressuresection of the steam turbine.
 8. In a combined cycle system including agas turbine, a steam turbine and a multi-pressure heat recovery steamgenerator, the improvement comprising means for supplying gas turbinecooling duty steam from a high pressure section of the steam turbine andfrom an intermediate pressure evaporator in the multi-pressure heatrecovery steam generator; means for conducting the gas turbine coolingduty steam to the gas turbine for cooling hot gas turbine pans; andmeansfor returning the gas turbine cooling duty steam to an intermediatepressure section of the steam turbine.
 9. The improvement of claim 8wherein a start-up circuit is provided which includes means forextracting steam from a fist pass of a high pressure superheater in themulti-pressure heat recovery steam generator; means for mixing theextracted steam with steam discharged from the high pressuresuperheater; and means for supplying the mixed steam to the gas turbinecooling system.
 10. The improvement of claim 8 and including means formixing the gas turbine cooling duty steam with steam from a reheater inthe heat recovery steam generator before the gas turbine cooling dutysteam is returned to the intermediate pressure section of the steamturbine.
 11. The improvement of claim 9 wherein means are provided forreturning gas turbine cooling duty steam to a condenser, bypassing theintermediate pressure section of the steam turbine.
 12. The improvementof claim 8 wherein the heat recovery steam generator includes amulti-pass superheater and wherein means are provided for controllingtemperature in the superheater by extracting steam from a first pass ofthe superheater, for conducting the extracted steam outside the heatrecovery steam generator, and for reintroducing the extracted steam atthe inlet end of the final superheater. .Iadd.
 13. In a method ofoperating a combined cycle system including a gas turbine combined witha steam cycle incorporating a steam turbine and a multi-pressure heatrecovery steam generator, wherein gas turbine exhaust is used in theheat recovery steam generator for reheating steam from the steamturbine, the improvement comprising supplying gas turbine cooling dutysteam from a high pressure section of the steam turbine and from anintermediate pressure evaporator in the multi-pressure heat recoverysteam generator, conducting the gas turbine cooling duty steam to thegas turbine for cooling hot gas turbine parts, and then returning thegas turbine cooling duty steam to the steam cycle; and wherein, early instart-up, prior to the availability of any cooling steam, the gasturbine hot stationary and rotating parts are cooled with filteredcompressor discharge air, which, after cooling the gas turbine hotstationary and rotating parts, is discharged to the gas turbine exhaust..Iaddend..Iadd.14. In a method of operating a combined cycle systemincluding a gas turbine combined with a steam cycle incorporating asteam turbine and a multi-pressure heat recovery steam generator,wherein gas turbine exhaust is used in the heat recovery steam generatorfor reheating steam from the steam turbine, the improvement comprisingsupplying gas turbine cooling duty steam from a high pressure section ofthe steam turbine and from an intermediate pressure evaporator in themulti-pressure heat recovery steam generator, conducting the gas turbinecooling duty steam to the gas turbine for cooling hot gas turbine parts,and then returning the gas turbine cooling duty steam to the steamcycle; and wherein, during start-up, as start-up cooling steam is beingestablished, the start-up cooling steam is bypassed around anintermediate pressure section of the steam turbine to warm the pipingand valves. .Iaddend..Iadd.5. In a method of operating a combined cyclesystem including a gas turbine combined with a steam cycle incorporatinga steam turbine and a multi-pressure heat recovery steam generator,wherein gas turbine exhaust is used in the heat recovery steam generatorfor reheating steam from the steam turbine, the improvement comprisingsupplying gas turbine cooling duty steam from a high pressure section ofthe steam turbine and from an intermediate pressure evaporator in themulti-pressure heat recovery steam generator, conducting the gas turbinecooling duty steam to the gas turbine for cooling hot gas turbine parts,and then returning the gas turbine cooling duty steam to the steamcycle; and wherein, during start-up, steam is extracted from a firstpass of a high pressure superheater in the multi-pressure heat recoverysteam generator, mixed with steam discharged from the high pressuresuperheater and then supplied to the gas turbine cooling duty system..Iaddend.